Display devices, such as liquid crystal displays (LCDs) and light emitting diode displays (LED-displays) are complex devices. In addition to the display medium, i.e. the liquid crystal layer forming the optical switch in LCDs and the electroluminiscent layer in LED-displays, display devices further comprise a large number of separate films and layers with various functions, such as display substrates and optically active components, such as polarizers, retarders, diffusers, reflectors, transflectors and color filters etc.
Presently, many such optically active components are laminated on the exterior of the display substrates. The actual optical active components are usually sandwiched between protective sheets or applied onto a carrier sheet. Further, adhesive layers are required in order to laminate the layers onto the display substrate. Through the use of these additional layers, each component adds ˜100 μm on the display thickness.
Work has been done to enable the use of polymeric substrates instead of conventional glass substrates. Polymeric substrates are desirable since they would allow for the manufacture of less brittle, low weight, flexible displays.
However, the intrinsic flexibility of displays that are based on polymer substrates is limited by the use of passive optical films with a thickness of ˜100 μm each, as the bending stiffness of the display scales with the third power of the thickness. Furthermore, several display device applications, such as LCDs using polarization optics for image formation, (such as twisted nematic, super-twisted nematic, in plane switching, vertically aligned nematic, optically compensated birefringence and ferro-electric LCDs) and polyLED-displays using a combination of a retarder and a polarizer for daylight contrast enhancement, require that the display substrates are isotropic, since birefringence in the substrate would impair the optical performance of the display as the polarization of light changes while traveling through a birefringent substrate.
Most polymeric materials exhibit birefringence and are thus not suitable for use as display substrates. A few non-birefringent, or nearly non-birefringent, polymeric substrates with retardation values below 10 nm, have been used with some success. However, upon applying a strain on these substrates, a significant change in their retardation occurs. Such strain might for example occur when a flexible display is being bent. This change in retardation impairs the optical performance of a display device using such substrates.
U.S. Pat. No. 6,151,088 proposes a liquid crystal display based on polymeric birefringent display substrates. A display according to the '088-patent requires that several requirements regarding angles between the polarizer axis, the optical axis of the substrate and the direction of the liquid crystal molecules in the middle portion of the liquid crystal layer. Furthermore the retardation value of suitable substrates is strictly limited. A display device according to the '088-patent is also bound to be unsuitably thick for flexible displays, as polarizers and other optical components are laminated on the outside of the substrate in the same manner as described above.
Thus, it would be advantageous to provide a display device than can utilize birefringent polymeric materials with low or no limitations on the retardation value as display substrates.